1. Technical Field
The present disclosure relates to a driving apparatus for an electric motor, in particular a brushless motor, and related method.
2. Description of the Related Art
It is known that direct current, electronically switching brushless motors may be driven either in “current mode”, i.e., by controlling the current crossing the motor windings, or in “voltage mode”, i.e., by controlling the voltage which is applied to the motor windings. In voltage mode driving systems, there is no feedback loop, and the voltage applied to the motor is directly proportional to the reference value multiplied by a given gain.
U.S. Pat. No. 6,137,253 describes a voltage mode driving method of a brushless motor; FIG. 1 shows a block chart of a driving system implementing the aforesaid and which is applied to a spindle motor 1. Energizing voltages PhaseA′-PhaseC′ 1, i.e., the voltages between the terminals A′, B′, C′ and ground GND shown in FIG. 2a along with voltage VCT at the star point CT of motor 1, are forced on the ends of the phase windings of the spindle motor 1, which voltages instead of having a given predetermined constant level during each step of switching, have a given profile (not a constant value) previously defined, digitized and stored in a static memory 2.
The system comprises a multiplier 3, adapted to multiple the profiles stored in memory 2 by a scale factor KVAL, a device 4 adapted to convert to PWM the profiles output by said multiplier 3 so as to obtain the PWMA-PWMC control signal for the respective half-bridges M1-M2, M3-M4, M5-M6 connected between voltage Vmotor and ground GND and adapted to drive the spindle motor 1. The terminal of a respective phase winding of spindle motor 1 is connected to each output node of the three half-bridges.
The system comprises a back-electromotive force or Bemf detector 5 adapted to detect the instantaneous position of spindle motor 1 to allow the correct synchronization of the PhaseA-PhaseC voltage switchovers applied to the motor by the rotor position. The system comprises a frequency multiplier 6 adapted to multiply the frequency output from the detector 5 by a factor equal to the number of samples with which the voltage profile contained in the memory may be discretized. The frequency output from block 6 is used by block 2 as a clock to scan the memory registers.
The register 7 contains the starting address from where to start scanning the profile stored in the memory at the next detection of the zero crossing of the Bemf.
The content of the register 7 and the KVAL factor may be inserted by means of a serial port 8. According to the KVAL value, the voltage value applied to the motor is modulated in amplitude from zero to its maximum value.
In a voltage mode-type, brushless motor driving system, the motor current is not directly controlled but depends on the voltage applied according to the relation:
      Vpha    ′    =            Rma      ×              Ipha        ′              +          Lma      ×                        ⅆ                      Ipha            ′                                    ⅆ          t                      +          Bemf_pha      ′      where Vpha′ is the differential voltage between the voltage PhaseA′ and the star point CT of the spindle motor 1, Rma and Lma and the resistance and the inductance of the winding of Phase A′ of motor 1, Ipha′ is the current in the winding of Phase A′ and Bemf_pha′ is the back-electromotive force of Phase A′, i.e., the back-electromotive force between terminal A′ and terminal CT. Voltages Vpha′, Vphb′ and Vphc′ are shown in FIG. 2b. 
Under normal operating conditions of a hard disk having spindle motor driven by a voltage mode type system, the differential voltage Vpha′ applied to the motor is generally always higher than the back-electromotive voltage Bemf_pha′ so as to generate a current which produces a positive torque in the motor. If the voltage Vpha′ is lower than the back-electromotive force Bemf_pha′, a current inversion occurs in the motor.
The current is generally supplied to the motor 1 from the external power through the power stage, e.g., the transistor half-bridges M1-M2, M3-M4, M5-M6 connected between voltage Vmotor and ground GND in FIG. 1; the normal current flow is directed from the external power to the motor.
If the voltage Vpha′ is lower than the back-electromotive force Bemf_pha′, the spindle motor acts as a generator and the current instead of being supplied by the external power is generated by the motor itself.
Under this condition, the current polarity is inverted with respect to normal operating condition and the current flow in this case is directed from the motor to the external power.
If the external power has a poor absorption capacity, the current is pushed back by the motor, i.e., there are no components connected to the power line capable of absorbing such a current, said current will therefore produce a voltage increase, the value of which will depend on the amplitude of the current and the duration of the driving step in which the back-electromotive voltage is higher than the voltage applied to the motor. This increase of power voltage, if not controlled, may be very dangerous for the application and cause breakage of the output stage which controls the motor due to overvoltage. During operation of a hard disk, this condition may be caused by a KVAL factor programming significantly lower than the previous one. The KVAL factor value is defined by the speed control routine of the hard disk: in order to keep the adjusting speed, the speed control system corrects KVAL according to the error between real speed and desired speed. Normally, these corrections are minor and do not produce any condition in which the voltage applied to the motor is lower than the value of the back-electromotive force. However, if the desired speed is changed to a lower value, the control loop reacts by decreasing the KVAL factor value, decreasing the voltage applied to the motor and causing the motor to slow down towards the new desired value. In such a case, the KVAL factor value programmed by the speed control circuit is such to provide the motor with a lower voltage as compared to the Bemf generated by the same, and thus until the motor has slowed down to the new desired speed, the current is inverted, because it is generated by the motor, and an overvoltage may occur on the power line.
FIG. 3 and more in particular FIG. 4 show what occurs when in a real application with speed control active, the desired speed is varied from 7200 rpm to 7100 rpm; the figures show the voltage PhaseA′ and the current Ipha′, the Target-Speed signal for the desired signal programming change, the power voltage Vmotor and the Zero-Cross signal, the value of which changes from positive to negative whenever the current crosses zero. When Target-Speed changes, the control loop based on an integral proportional filter with Kp and Ki constants, for example, reacts thus gradually decreasing the KVAL factor value. When the KVAL factor value is such that the back-electromotive voltage of the motor exceeds the voltage applied to the motor, the current polarity is inverted and instead of flowing from the external power supply to the motor, it flows from the motor to the power supply, thus causing a voltage increase on the Vmotor line, which from initial 12V is raised to 16V, as better shown in FIG. 4, which is a detailed part of the time chart in FIG. 3; there is a current inversion in the motor and the subsequent voltage increase on the Vmotor line caused by a decrease of the programmed KVAL factor by the speed control which reacts to the desired speed decrease.
The voltage increase on the Vmotor line then gradually disappears when the real speed reaches the new desired speed, re-establishing a situation in which the current deriving from the external power supply is driven in the motor by the control circuit. The voltage increase on the Vmotor line, generated by this operating condition, is absolutely not controlled and may be influenced by various factors, such as the extent of the desired speed decrease, the value of Kp and Ki coefficients of the integral proportional filter of the speed control loop, the electromechanical features of the motor (Rm, Lm, inertia, etc.).